Conventionally, in radiation treatment planning, information about an Internal Target Volume (ITV) (i.e., a region in which a region of interest (“Clinical Target Volume [CTV]”) such as a tumor can move around during a respiratory cycle) has been considered important in determining a region to which radiation is to be applied. It is desirable to arrange an ITV to be as small as possible, to reduce the burden on a patient.
As for methods for estimating an ITV, examples of known methods include a dynamic volume image taking process and a respiration synchronized image taking process that employ an X-ray Computed Tomography (CT) apparatus such as an Area Detector CT (ADCT) or a multi-row detector CT. During such an image taking process, it is possible to view movement states of a tumor at multiple points in time during a respiratory cycle, together with movements of structures that are present in the surroundings of the tumor. According to this conventional technique, however, it is not possible, in some situations, to make clear the relationship between the regions used for estimating the ITV and the respiratory states.